The Republican leader of the US Senate, Mitch McConnell, delivered a rare rebuke to Donald Trump in legislation that questions the president's decision to pull US troops out of Syria.

The amendment to a broader Middle East Policy bill easily passed a preliminary procedural vote on Thursday 68 to 23, with substantial support from both Democrats and Republicans.

McConnell, who rarely criticises Trump openly, said earlier this week that the amendment "would acknowledge the plain fact that al-Qaeda, ISIS, and their affiliates in Syria and Afghanistan continue to pose a serious threat to our nation".

"It would recognise the danger of a precipitous withdrawal from either conflict and highlight the need for diplomatic engagement and political solutions to the underlying conflicts in Syria and Afghanistan," he said.